This invention relates to an oxygen-permeable hydrophilic contact lens and its method of preparation.
Presently, contact lenses are made from optically clear substances which may be either rigid or flexible and may or may not be hydrophilic. Though the most common contact lenses are intended to cover only the cornea, and thus have a diameter of about 9 mm., scleral lenses embodying this invention may prove ultimately more convenient. Scleral lenses can be made with diameters as great as 2.0 cm. The most commonly used substances have been mineral glass or clear plastic materials such as polymethyl methacrylate. While these substances are optically satisfactory, their use is disadvantageous since they are substantially impermeable to oxygen. Since the cornea is an avascular organ and depends upon oxygen diffusing to it from the atmosphere for its vitality, frequent removal of the impermeable contact lens is necessary so that the cornea may remain viable. In addition, contact lenses made from glass or rigid polymeric material may not be fitted precisely over the cornea or may not be wettable sufficiently by water so that an air gap may develop between the cornea and the contact lens producing drastic aberration in the optical path because of the discontinuity of the interface between the cornea and the contact lens. To overcome this latter problem, hydrogels containing substantial quantities of water, in excess of 30 or 40% by weight, have been employed. These lenses have the advantage of optical clarity and flexibility making them more comfortable during use. However, their permeability to oxygen is relatively poor so that even though comfortable, they must be removed frequently to permit contact of the cornea with oxygen.
It has been proposed to form a contact lens from an oxygen-permeable silicone core which is rendered hydrophilic by grafting a hydrophilic vinyl monomer to its surface. British Pat. No. 1,170,810 and corresponding French Pat. No. 1,526,934 to Laizier et al. disclose a process for rendering the surface of a silicone contact lens hydrophilic by irradiating uncross linked silicone in the presence of free oxygen to crosslink the silicone while forming peroxide radicals therein (especially at the surface) and subsequently immersing the crosslinked silicone in a vinyl monomer at an elevated temperature for a period of about one-half hour to about two hours. The monomer is grafted to the silicone by free-radical polymerization chemically initiated by the previously formed peroxide. This process is time-consuming and the contact lens produced by this process is undesirable for a number of reasons. First, the grafting polymerization must be conducted at temperature levels and over periods of time under which the vinyl monomer is capable of periods of time under which the vinyl monomer is capable of diffusing into and swelling, previously grafted polymer chains, thereby producing irregular regions of proliferating grafting by a positive feedback effect. In other words, after the very first grafting to the hydrophilic silicone surface has been accomplished the new surface is much more absorptive to the monomer. The monomer is thermodynamically compelled to swell the grafted hydrophilic polymer and further irradiation will produce additional grafting in regions already reached in the monomer. Thus the process is one of positive feedback and can lead to gross differences in amount of grafted hydrophilic polymer per unit area of silicone substrate. Upon subsequent immersion in water the more grafted regions swell to the greater degree, thereby distorting the interface and producing undesirable optical dispersion. Secondly, reliance only on peroxide radicals to vitiate grafting is undesirble since the peroxide groups may subsequently undergo cleavage, releasing the grafted hydrophilic monomer.
The Lazier et al patent also notices that prior attempts to irradiate mutually a silicone and N-vinyl pyrrolidone have resulted in unsatisfactory products since the optical characteristics of the silicone were not retained after irradiation.
Accordingly, it would be highly desirable to form a contact lens structure which is flexible, optically clear, water wettable and highly permeable to oxygen, by a more simple and controllable process. Such a lens would be comfortable to wear, permitting natural lubrication of tear fluid against the moving eye lid, would ensure gas-free contact with the cornea at the cornea-lens interface and would eliminate the need for frequent removal of the contact lens to permit contact of the cornea with oxygen.